Vision Reduction and Loss - Understanding Causes and Points for Timely Action

Reduced vision, blurring or loss of vision (blindness) is the primary concern for a person with any eye problem as well as for the doctor. One should know the possible causes affecting vision, when to take timely action and seek immediate consultation from the eye specialist.

The commonest causes of blurred vision are refractive errors (eye number) so that should be tested.

Read about: REFRACTIVE ERRORS

The structure and parts of the eye along with the causes of vision loss are summarized in the figure below.

STRUCTURE OF THE EYE

The eyes are actually 2 eyeballs placed in bony sockets called orbits on either side in the skull and coverable by eyelids. The eyeball consists of the transparent cornea in the central front, and the white sclera all around. The exposed part of the sclera in front adjoining the cornea, is covered by a thin membrane layer called the conjunctiva. Extraocular muscles called the recti and the obliques facilitate coordinated eye movement in all directions and maintain eye alignment, and their lack of proper function can lead to squint (strabismus).

Behind the cornea is the iris with a central opening called the pupil. The diameter of the pupil is reduced in bright light (constricted) and increased in dim light (dilated), and this is controlled by the sphincter iris muscle. The iris contains blood vessels and pigment that gives the characteristic ‘eye color’. The iris leads behind into the ciliary body which suspends the lens of the eye just behind the cornea and pupil, with the help of fibers called zonules. Contraction of the ciliary muscle makes the lens more biconvex (fatter or thicker) to view near objects clearly. This is called accommodation which helps in reading and doing near work. Loss of transparency of the lens with age is called cataract.

The space between the cornea and iris is called the anterior chamber, and that between the iris and lens is called the posterior chamber. These chambers are connected and are filled with a nourishing fluid called aqueous. The aqueous drains out of the eye through the angle of the anterior chamber which contains a sieve-like structure called the trabecular meshwork (TM). Any reduction in aqueous drainage due to narrowing of the angle or abnormality in the TM, can lead to an increase in eye pressure, called glaucoma

The ciliary body continues behind as the choroid which forms the layer behind the retina. The iris, ciliary body, and choroid are together called the uveal tract or uvea The space between the lens and the retina is filled with vitreous (a gel-like transparent substance).

The retina is the innermost back layer of the eye and has an outer pigmented layer and an inner light-sensitive layer (neural retina). The neural retina has photoreceptors called rods (perceive contrast) and cones (perceive color), which are especially present in high density in the central area of the retina called the macula. The macula is responsible for the central, high-resolution, and color vision. The retina receives visual signals and sends them through the optic nerve (seen on the retina as the optic disc or optic nerve head), to the brain for the interpretation of vision. The pigment layer of the retina contains protective carotenoid xanthophyll pigments lutein and zeaxanthin, the latter predominating in the macula.

The front 1/3^rd of the eye in front of the vitreous is called the anterior segment, while the back 2/3rd is called the posterior segment. The anterior segment includes the cornea, iris, ciliary body, zonules, lens and both anterior and posterior chambers (filled with aqueous). While the posterior segment includes the vitreous, choroid, retina, and the emerging optic nerve. (So, the ‘chambers’ and ‘segments’ of the eye should not be confused!)

EYE EXAMINATION

The slit lamp is an instrument used to examine the eye in detail as it gives an enlarged and in-depth view of the cornea, anterior chamber, iris, and the lens.

For performing an effective eye examination and to visualize the eye behind the iris for better evaluation of cataract and the condition of the vitreous, retina, and optic nerve, the pupil of the eye has to be dilated with suitable eyedrops. This may temporarily cause glare and blurring of near vision, till the drug effect wears off in a few hours.

To examine the retina, an instrument called the ophthalmoscope or fundoscope is used. These are of two types – direct and indirect ophthalmoscope, with the latter giving a better view of the peripheral retina, and the former, a better view of the optic disc and macula.

WHEN TO SEEK IMMEDIATE GUIDANCE OF EYE SPECIALIST

This should be done if any of the following sight-threatening signs and symptoms appear:

• Sudden blurring, reduction, or loss of vision
• Curtain falling/ red ink filling up perception
• Seeing flashes of light
• Seeing blurred/blind spots
• Seeing a large number of floating spots/webs
• Seeing colored haloes around lights
• Increased sensitivity to light or glare
• Seeing double images
• Sudden severe pain in or around the eyes
• Pain on eye movement
• Severe red eyes with swollen lids
• Thick discharge coming from the eye
• Change in color of eyes or white spots

Regular eye screening and check-up are advised in the following conditions:

• Diabetes
• Hypertension (high BP)
• Cardiovascular disease
• Age>60
• Family history of eye diseases

EYE CONDITIONS CAUSING REDUCTION OR LOSS OF VISION

The common eye conditions and diseases that cause a decrease in or loss of vision are listed below.

CATARACT

GLAUCOMA

CORNEAL INFECTIONS AND KERATITIS

UVEITIS

CONDITIONS OF RETINA AND OPTIC NERVE

OCULAR TRAUMA

Apart from this eye injury or ocular trauma is the most common cause of sudden loss of vision. An eye specialist should be immediately consulted in case of any trauma to the eye, as urgent intervention/surgery may be needed in some cases. Eye trauma causing loss or reduction of vision can be due to:

• corneal injury or perforation
• hemorrhage or bleeding
  • anterior chamber hemorrhage (hyphema)
  • vitreous hemorrhage
• dislocation of the lens
• detachment of a part of the retina
• traumatic optic neuropathy

Note: Blindness after head injury can also be due to damage to the vision area of the brain.

Further read:

Cataract and Cataract Surgery – 3 Important Points

Red Eyes – Keratitis and Scleritis

Glaucoma: What you need to know

Uveitis – 5 Important Points of Understanding

Retina and Optic Nerve Diseases – Overview and Types

For any query, additional information or to discuss any case, write to info@drvarsha.com and be assured of a response soon

References

Vision Loss -AAOS